MXPA97003602A - Process for the removal of metallic impurities through electroquim route - Google Patents

Abstract

A method for electrochemically purifying solutions with a pH of 14 to reduce metallic impurities at the trace level. The method comprises the processing of a solution in an electrolytic cell, wherein the cathode has fibrous network produced from a mixture of fibers, including at least one fraction consisting of electrically conductive fibers, and a selected binder of fluropolymers, wherein the network fibrous is deposited on a porous electrically conductive support. The cathode can also be combined with a diaphragm or membrane. It also reveals a cathode with a fibrous network produced from a mixture of carbon fiber, cellulose compounds and a cationic polymer, such as cationic starch.

Description

PROCESS FOR THE REMOVAL OF METAL IMPORTS THROUGH ELECTROCHEMICAL ROUTE The present invention relates to a process for the purification of solutions whose pH is greater than 14, by means of an electrochemical route, with the aim of reducing metal impurities in the form of traces. In particular, the process according to the present invention is suitable for the purification of solutions of alkali metal hydroxides, whose pH is higher than 14. In addition, the present invention relates to a particular cathode capable of being applied in the process of according to the invention. The electrochemical methods to obtain solutions whose pH is higher than 14, and in particular of solutions of alkali metal hydroxides, are now well known and industrially developed. The electrolysis in diaphragm cells can be mentioned among the electrochemical methods to obtain these solutions. Similarly, electrolysis in mercury cells is known. This process consists, in a first step, in the production of a sodium and mercury amalgam from a solution saturated with sodium chloride and then, in a second step, in reacting the amalgam with water to produce sodium hydroxide. Finally, electrolysis can be mentioned in a membrane cell, where the anode and cathode compartments do not communicate with each other. These latter two methods make it possible to produce solutions of alkali metal hydroxides that contain relatively few metallic impurities. In fact, in contrast to the diaphragm cells, there is practically no corrosion due to the reaction medium in the electrolyzer itself. In the case of mercury cells, the anode and cathode corrosion problem does not arise because the anode is of mercury, and the graphite cathode, or vice versa, according to the compartment of the cell. In the case of membrane cells it should be noted that the construction materials of the membrane cells are much more resistant than those that are generally used for diaphragm cells: the membrane is very brittle, and therefore should be contaminated as little as possible. possible with metals produced by corrosion of cell materials, and if this fails, the operation of the cell is considerably reduced. Finally, the subsequent stages of concentration of the soda are lower in the case of mercury and the membrane cells, in the case of the diaphragm cells, because the soda obtained is more concentrated. This proportionally decreases the corrosion risks of the equipment used for the concentration. Accordingly, the alkali metal hydroxide solution produced in diaphragm cells is contaminated by the presence of metallic elements originating from the electrodes, and also from the equipment used during the concentration stage. These metals appear in an amount of the order of a few parts per million, a content that is higher than that of the alkali metal hydroxide solutions that originates from the other two types of cells that were mentioned. It is therefore necessary to have available solutions whose pH is higher than 14, and in particular alkali metal hydroxide solutions, which have a degree of purity related to these metals, comparable with the hydroxide solutions obtained by the electrolysis in mercury and cells of membrane. In addition, it is necessary to perform a purification of these solutions using methods that are simple and economical. There are methods for the electrochemical purification of alkali metal hydroxide solutions. They generally use containers including, as a cathode, hollow graphite cylinders whose porosity is controlled. The disadvantage of a process of this type lies in the manufacture of these cylinders, and in the fact that the equipment is too large. In fact, the cross section of the electrode is 5 m2 / m3 of the working volume of the electrode. The subject of the present invention is therefore a process for the purification of solutions whose pH is greater than 14., that does not present the aforementioned disadvantages. Accordingly, the process according to the invention consists in treating solutions whose pH is greater than 14 in an electrolysis cell, wherein the cathode includes a fibrous sheet based on a mixture of fibers, where at least a proportion thereof is electrically conductive , and of a chosen agglutinator of fluoropolymers, where the fibrous sheet is deposited on a porous electrically conductive support. The solutions that can be treated according to the process of the invention are in particular alkali metal hydroxide solutions having a concentration of 40 to 800 alkali metal hydroxide per liter of solution. The method according to the present invention makes it possible to obtain solutions having metal impurity contents of less than 1 mg / kg, or even 0.01 mg / kg.

One of the advantages of the process according to the invention is that the volume of the containers is up to 4 a times smaller than the one that represents the aforementioned purification process. In addition, the regeneration of the cathodes used is very simple and effective. In addition, these cathodes are highly resistant despite the extremely high pH values of the solution to be treated. However, other features and advantages may be seen more clearly by reading the following description and examples. Accordingly, the process according to the invention is used in an electrolysis cell, wherein the cathode includes a fibrous sheet consolidated in a fluoropolymer, and deposited in an electrically conductive support. The fibrous sheet of the cathode, also known as electrically conductive microporous material, conducts electricity. In particular, it has an electrical resistivity between 0.5 and 15 O cm. The fibrous sheet is obtained from a mixture of fibers, wherein at least a fraction of the fibers is electrically conductive, optionally in combination of non-conductive fibers. The electrically conductive fibers can thus be fibers that are intrinsically conductive, or at least treated to make them so. According to a particular embodiment of the present invention, entrinsically conductive fibers such as, especially, carbon or graphite fibers are used. In particular, these fibers are in the form of filaments whose diameter is generally less than 1 mm and in particular between 10 ~ 3 and 0.1 mm, and whose length is greater than 0.5 mm and more especially between 1 and 20 mm. In addition, the conductive fibers preferably have a monodisperse length distribution, i.e., a distribution such that the length of at least 80%, and advantageously of at least 90% of the fibers corresponds to the average length within a range of ± 10%. Among the non-conductive fibers that are optionally used, two categories can be recognized: organic fibers and inorganic fibers. A first class of organic fibers consists of polypropylene and polyethylene fibers, or between fluoropolymer fibers.

"Fluoropolymers" is intended to mean homopolymers or copolymers derived from at least a portion of olefin monomers substituted with fluorine atoms or substituted with a combination of fluorine atoms and at least one chlorine, bromine or iodine atom, per monomer. Examples of fluoro homopolymers or copolymers may consist of polymers and copolymers derived from tetrafluoroethylene, hexafluoropropylene, chlorotrifluoroethylene or bromotrifluoroethylene. These polymers can also include up to 75 mol% of units derived from other ethylenically unsaturated monomers containing at least as many fluorine atoms as carbon atoms, such as, for example, phenylidene difluoride and vinyl and perfluoroalkyl esters such as pertfluoroalkoxyethylene. The organic fibers made of polytetrafluoroethylene (which will be called PTFE fibers in the following) are preferably used. Organic fibers, and in particular PTFE fibers, generally have a diameter of between 10 to 500 μm and a length such that the ratio of length to diameter is between 5 and 500. Organic fibers whose average dimensions are between 5 and 200 μm in the case of the diameter and, between 1 and 10 mm in the case of the length, are those that are used preferably. A second category of organic fibers consists of compounds based on cellulose fibers. These compounds are preferably used with a cationic polymer, such as cationic starch. These compounds can thus be pretreated with this cationic polymer, or else this polymer can be added to the fiber mixture independently of the compounds based on the cellulose fibers. Fibers that were given a positive ionic charge can be used, especially treating these fibers with a polymer or cationic starch. It is also possible to use fibers with a positive surface charge, marketed by the Beco company under the name Becofloc®. The fibers of the first category can, of course, be used in combination with the fibers of the second category. In particular, PTFE fibers can be used in combination with the aforementioned cellulose-based compounds. The inorganic fibers are chosen from ceramic fibers such as zirconium dioxide, silica carbide and boron nitride fibers, or titanium suboxide fibers or titanium suboxide fibers of the general chemical formula Tin02n-? where n, an integer, is between 4 and 10 (of the Ebonex® type, which are manufactured and marketed by the company Atraveda). The mixture of the fibers additionally includes a fluoropolymer that consolidates all the fibers. The definition given above for polymers of this type remains valid in this case, and therefore will not be repeated in the present. This fluoropolymer, or agglutinator, more particularly has the form of an aqueous dispersion containing from 30 to 80% by weight of dry polymer, whose particle size is between 0.1 and 5 μm and preferably between 0.1 and 1 μm. According to the particular embodiment of the present invention, the fluoropolymer is polytetrafluoroethylene. Also, it should be noted that the mixture from which the fibrous sheet is obtained can include other additives such as, especially, thickening or pore forming agents and surfactants. As far as pore-forming agents are concerned, any compound is suitable, as long as it can be removed, for example by decantation or thermal decomposition. However, silica-based derivatives are preferably used. These compounds are particularly advantageous because they practically do not disintegrate the microporous electrically conductive material, and form networks with the polymer, binding the fibers when the latter is used in the form of a latex. By "silica-based derivatives" is meant, according to the present invention, precipitated silicas and fumed or pyrogenic silicas. In particular, they have a BET specific surface of between 100 m2 / g and 300 m2 / g and / or a particle size, evaluated with a Counter® counter, between 1 and 50 μm and preferably between 1 and 15 μm. As surfactants, nonionic compounds, such as ethoxylated alcohols or fluorocarbon compounds containing functionalized groups, which generally have carbon chains containing from 6 to 20 carbon atoms, are more particularly used. The ethoxylated alcohols chosen from the ethoxylated alkylphenols, such as especially the octoxins, are those which are preferably used. Thickening agents are intended to mean compounds capable of increasing the viscosity of the fiber mixture, and having water retention properties. Generally, natural or synthetic polysaccharides are used. Special mention may be made of the biopolymers obtained by fermentation of a carbohydrate under the effects of microorganisms, such as xanthan gum or any other polysaccharide having similar properties. As already indicated above, the electrically conductive material is deposited on a porous support. Fabrics or networks whose interwoven gap, perforations or porosity, can be between 20 μm and 5 mm, and are especially suitable. These porous supports may have one or more sill or flat surfaces, commonly known as "thimbles", which have an open surface. The cathode used in the process according to the invention, which corresponds to the porous support and the fibrous sheet unit, has a large specific surface area, necessary to obtain good results (the cross section of the electrode is of the order of 20 to 50 m2 / m2 of working volume of the electrode), as long as it is of a small m This presents an undoubted advantage in the case of an industrial exploitation. A particular cathode, which also forms part of the subject of the present invention, is used in the process according to the present invention. This cathode includes a fibrous sheet obtained from a mixture of carbon fibers, compounds based on cellulose fibers, and a cationic polymer, as in the case of a cationic starch. The cathode according to the present invention can similarly include a fibrous sheet obtained from a mixture of carbon fibers, and from compounds based on cellulose fibers pretreated with a cationic polymer, such as cationic starch. Also, the fiber blend preferably includes a pore-forming agent such as silica, as well as a surfactant. According to an alternative form of the invention, and highly advantageous, the fibrous sheet deposited in the porous electrically conductive support is used in combination with a microporous diaphragm. In accordance with a first embodiment of this alternative form, the diaphragm is deposited on the fibrous sheet, especially in accordance with a preparation process described below. These diaphragms generally consist of a fibrous sheet, including a mixture of organic or inorganic fibers with a fluoropolymer that binds these fibers. The organic fibers that may be mentioned are polyethylene, polypropylene or fluoropolymer fibers, as well as cellulose-based fibers. The inorganic fibers that can be used are especially carbon, graphite, ceramics, titanate and titanium suboxide fibers. All these were mentioned above in regard to the aforementioned fibers, as well as the additives which are suitable for the preparation of the fibrous sheet, and which are valid as far as the diaphragm is concerned, and therefore they will not be repeated. The diaphragm used in combination includes a mixture of organic fibers and inorganic fibers which are preferably chosen from carbon fibers, graphite and titanate fibers. According to a second embodiment of this alternative form, the diaphragm is not deposited on the fibrous sheet but is arranged separately to separate the compartments of the anode and the cathode. These diaphragms are commercially available, and are based especially on ceramic or Teflon type fibers. According to a second alternative form of the invention, the cathode, including the fibrous sheet deposited on an electrically conductive support, is used in combination with a membrane.

Examples of membranes suitable for the process according to the present invention, and which may be mentioned, are perfluorosulfonic membranes of the Nafion type (marketed by the company Du Pont), or perfluoro membranes that include carboxylic functional groups (series Fx-50 or 890, marketed by the company Asahi Glass).

In addition, it is possible to use two-layer membranes, including sulphonic groups on one side, and carboxyl groups on the other. A method of preparing the cathode will now be described. The cathode according to the present invention can be obtained by a wet route, by depositing, under a programmed vacuum, a suspension that includes the constituent elements of the fibrous sheet, through a porous support. In particular, the process of preparing the cathode consists in carrying out the following steps: [a] an aqueous suspension is prepared, including a mixture of fibers wherein at least a fraction consists of electrically conductive fibers, a chosen agglutinator of the fluoropolymers, a pore-forming agent and, if appropriate, additives; [b] the suspension is deposited on a porous support by filtration under a programmed vacuum; [c] the sheet obtained in this way is drained and optionally dried; [d] the resulting unit is agglomerated at a temperature that is greater than or equal to the melting or softening temperature of the binder, [e] the pore-forming agent is removed by a treatment carried out before or after the use of the cathode. As mentioned above, the first stage of the process consists in the preparation of a suspension, including the constituent components of the fibrous sheet. According to a particular embodiment of the present invention, the suspension includes a mixture of 20 to 80 parts by dry weight of conductive fibers, preferably of to 40, and 20 to 100 parts by dry weight of non-conductive organic fibers. From 0 to 50 parts, by dry weight, cationic polymer can be used, as in the case of cationic starch. The suspension may additionally include 10 to 100 parts by dry weight of non-conductive inorganic fibers. In particular, the suspension may include from 20 to 60 parts by dry weight of non-conductive inorganic fibers.

The non-conductive inorganic fibers are in particular chosen from the titanate fibers. The fluoropolymer content that binds the fibers is generally between 10 to 60 parts by dry weight. The suspension used in the process according to the present invention generally includes from 30 to 200 parts by dry weight of at least one pore-forming agent. According to a particular method, the content of the pore-forming agent is between 30 to 100 parts by dry weight. The content of the thickening agent generally varies between 0 and 30 parts by dry weight. In particular, it is from 0 to 10 parts by dry weight. Finally, in most cases the suspension includes no more than 10 parts by dry weight of at least one surfactant, and in particular a content of between 0.5 to 5 parts by dry weight. In accordance with the process for preparing the cathode, the fibrous sheet is formed by filtering the suspension through a porous support under a programmed vacuum. This porous support may or may not be electrically conductive. If the second case is applicable, then before the agglomeration step [d] the sheet is separated from the non-conductive porous support, and applied to the conductive porous support before the whole is agglomerated. According to a preferred alternative form, the vacuum filtration of the suspension obtained in step [a] is carried out directly through the conductive porous support. The sheet is deposited on the porous support by filtration under a programmed vacuum. The latter is produced in a manner known per se, and can also be produced continuously or in stages, until reaching a final partial vacuum of 1.5xl03 to 4xl04 Pa. Once the sheet is deposited, drained by holding the vacuum for a few moments, and then optionally air-dried, at a temperature between room temperature and 150 ° C. The sheet is then agglomerated upon heating to a temperature greater than or equal to the melting temperature of the fluoropolymer. During this agglomeration step, a proportion of the constituents of the mixture from which the fibrous sheet is formed is generally thermally degraded. Next, a step of removing the pore-forming agent is carried out, especially by an aqueous solution of an alkali metal hydroxide.

It should be noted that the removal of the pore-forming agent can be done not only before the use of the electrically conductive microporous material, but also in situ, that is to say during the first moments of the use of the cathode. However, given the purpose for which the cathode is used, it may be preferable to prevent the solution from being contaminated, so that it is purified with the pore-forming agent dissolved during this step. In the case where the cathode used in the process according to the invention includes an associated diaphragm, in the sense that the diaphragm is deposited on the fibrous sheet, the following steps are carried out: [a] an aqueous suspension is prepared including a mixture of fibers, wherein at least a fraction consists of electrically conductive fibers, a chosen agglutinator of fluoropolymers, a pore-forming agent, and if appropriate, additives; [b] the suspension is deposited on a porous support by filtration under a programmed vacuum. [c] the sheet thus obtained is drained and optionally dried; [d] the sheet is optionally agglomerated at a temperature greater than or equal to the melting or softening temperature of the agglomerator; [e] optionally, the pore-forming agent is eliminated; [f] an aqueous dispersion, including at least one mixture of organic and inorganic fibers, an agglomerator, a pore-forming agent and, if appropriate, additives, which are filtered in the resulting sheet; [g] the unit formed in this way is drained and optionally dried; [h] the unit is agglomerated at a temperature greater than or equal to the melting or softening temperature of the agglomerator; [i] the pore-forming agent is removed by a treatment formed before use of the cathode, or when the latter is used. All that was said above, in relation to stages [a] to [e], remains valid and will not be repeated in the present part. The dispersion prepared in step [f] therefore includes a mixture of organic and inorganic fibers. Titanate fibers, carbon fibers or graphite, or a mixture of these, are used as inorganic fibers. In the case where carbon or graphite fibers are used, the content is in particular at least 2% by weight of the diaphragm. The content of graphite or carbon fibers preferably does not represent more than 10% of the weight of the diaphragm. Polytetrafluoroethylene fibers are used as organic fibers according to a particular embodiment. According to a particular embodiment, the dispersion includes a mixture of inorganic and organic fibers, whose content is between 30 and 80% by weight. In this mixture, the proportion of inorganic fibers represents 1 to 80% of the weight of the fiber mixture. The dispersion in step [f] additionally includes an agglomerator, whose amount may vary between 3 and 35% by weight. The dispersion includes a pore-forming agent, the amount of which is particularly between 5 and 40% by weight. Finally, the dispersion may include additives such as thickener surfactants, in a proportion that generally ranges between 0 and 5% by weight. It should be noted that the agglomeration step described in step [d] is not mandatory, while the agglomerator that forms part of the composition of each of the two sheets is identical, or has a melting temperature of the same order of magnitude. In fact, in this case, the agglomeration of the combination of the two sheets can be advantageously carried out after the deposition of the diaphragm. The solutions that must be purified according to the process of the present invention have a pH greater than 14.

The process according to the present invention is especially suitable for the purification of alkali metal hydroxide solutions. In this case, the solution to be treated is preferably obtained by electrolysis of a solution of its corresponding alkali metal halide. In particular, the invention is advantageous for the purification of solutions that originate from the electrolysis carried out in a diaphragm cell. Also, it should be noted that it should not constitute a separation from the scope of the present invention to purify a solution of an alkali metal hydroxide resulting from another method of preparation, provided that the problem due to the presence of metals arises in similar terms. . According to a preferred method of the present invention, the solution to be treated is sodium hydroxide solution.

The metals that may be present in the hydroxide solutions to be purified are generally chosen from iron, nickel, aluminum, chromium, vanadium, arsenic, selenium, lead, cadmium, manganese and copper. Of course, this list is not considered exhaustive. As a general rule, the content of metals in the solution to be treated "is not more than a few hundred milligrams per kilogram of solution.The process according to the invention can be used to purify solutions whose alkali metal acid content is 30. 50% by weight, although contents outside these ranges are not excluded In the particular case in which the alkali metal hydroxide solutions are obtained by electrolysis as formulated above, pretreatment of the solution is preferably carried out Before applying the purification process, therefore, the solution leaving the electrolysis cell, consisting of a mixture of alkali metal hydroxide and an initial halide, is concentrated, and the precipitated halide salt is separated from the solution This operation occurs according to methods that are known to those skilled in the art, for example by evaporators.

According to an alternative form, once the concentration is carried out, a desalting of the resulting solution is carried out, especially by reaction with an aqueous ammonia. The solution to be treated is then introduced into an electrolysis cell including the cathode described above, and an anode. The latter is generally made of nickel, nickel oxide or stainless steel. The cathode can be found both downstream and upstream of the anode. In the first case, two application systems are particularly suitable in the case of the cell, the agglomeration cells, or the cartridge cells. Regarding this last type of cell, the cathode has a flat shape. If the cathode is used in combination with a diaphragm which is not obtained by deposition of the latter on the cathode sheet, an alternative form of the process according to the invention is to use a commercial type diaphragm, for example, a diaphragm based in ceramic fibers or in Teflon. In this case, the diaphragm is adjusted in the cell downstream or upstream of the anode. The flow rates of the solution to be purified vary over a wide range, and depend on the amount of solution to be purified, and the ability of the electrolysis cells to treat the latter. The applied currents are generally less than 2000 A / m2. The duration of the purification cycle is advantageously in the order of 500 to 1000 hours. The cathodes used in the process of the invention can be regenerated by known means. This regeneration may, in fact, be necessary because during the treatment the contaminating metals are precipitated at the cathode, and this may mean a change in the permeability of the latter.

According to this, when the pressure necessary to obtain the same flow becomes too high, it is generally necessary to regenerate the cathode to make it reusable. This regeneration can be performed electrochemically. Thus, by inverting the polarity of the electrodes, or by reducing the value of the voltage at the cathode, a redissolution of the contaminating metals can occur.

The chemical method can be used, that is, using an acid of the hydrochloric, sulfuric or nitric type, or a base such as sodium hydroxide. Purified in this way, the solution has a content of metal impurities of not more than 1 mg / kg and can even be 0.01 mg / kg. In this way, and especially in the case of alkali metal hydroxide solutions, the solutions obtained are made suitable for subsequent uses such as the preparation of sodium hypochlorite, in which case the color of the iron is not desired. Similarly, the solutions obtained can be used in food applications, and for the preparation of phosphates. However, other advantages and features will be apparent upon reading the non-limiting examples of the invention, which are set forth below. EXAMPLES In the following, the electrolysis cell in the following characteristics: - Anode: expanded nickel, - Porous cathode support: stainless steel composed of 2 mm wires and 2 mm nets; - Cells assembled according to the type of filter press, - Cross section of the active surface of the cathode: 10 EXAMPLE 1 1 / Preparation of the cathode A suspension is prepared with stirring of the following compounds: demineralized water, whose amount is calculated in order to obtain approximately 4 liters of suspension and a solids content of approximately 4.8% by weight, g of PTFE fibers introduced in the form of a mixture of sodium chloride and PTFE fibers (50/50 by weight). These PTFE fibers impregnated with sodium chloride are obtained previously by mixing one liter of water, with stirring, with approximately 100 g of a mixture containing approximately 50% in PTFE fibers and 50% NaCl, - 70 g of carbon fibers ( average length 1.5 mm, approximate diameter 10 μm), - 15 g of PTFE in the form of latex with a solids content of approximately 65% by weight, - 100 g of precipitated silica (Rhóne-Poulenc Tixosil®, surface BET 250 m2 / g), - 9 g of xanthan gum. The mixture thus obtained is deposited by filtration on a porous support of 10 cm 2, consisting of a coiled and interlocked iron net, whose opening is 2 mm, and the diameter of the wire is 2 mm. The filtration is carried out under a programmed vacuum of the following formula: - 1000 Pa min "1 for 10 minutes, - 5000 Pa min" 1 to reach a final partial vacuum of 25 000 Pa. Then the whole is dried for 12 hours at 100 ° C, where the deposited weight is 0.45 kg / m2. A second suspension is prepared, which includes: - 100 g of PTFE fibers introduced in the form of 200 g of a mixture of fibers and sodium chloride (50/50 by weight) treated as described above, - 20 g of fibers of potassium titanate (diameter between 0.2 and 0.5 μm, length from 10 to 20 μm), - 20 g of PTFE in the form of latex with solids contents of approximately 65% by weight, - 30 g of precipitated silica (Rhóne-Poulenc Tixosil ®, BET surface: 250 m2 / g), - 3.6 g of triton (Rohm and Haas), 5 g of carbon fibers (length: 1.5 mm, diameter: 10 μm). The suspension is stirred for 30 minutes, and after settling for 48 hours, the suspension is again agitated and filtered on 1 dm2 of dry percatodo sheet previously obtained. The filtration is carried out under a programmed vacuum of 5000 Pa min "1, to reach 80,000 Pa. The compound thus obtained is dried for 12 hours at 100 ° C, and consolidated by melting the fluoropolymer at 350 ° C for 7 minutes The weight deposited in the case of the second sheet is 1. 5 kg / m2. 2 / Soda purification cycles, cathode regeneration The treated effluent is soda at a 50% desalinated concentration by treatment with aqueous ammonia. The treatment temperature to purify the soda is 50 ° C. The solution to be treated is introduced into the compartment of the anode of the cell before percolation through the cathode obtained in section 1 /. In this case, the configuration is such that the anode is upstream of the cathode. The electrode is regenerated when the hydraulic head becomes too high to impose a flow regime of the treatment. The regeneration is now necessarily complete and corresponds to the dissolution of the deposited metallic species. It is carried out by soaking percolation at 50% to 80% for approximately 24 hours, with a flow regime that varies between 150 1 / h m2 and 1000 1 / h m2 at the end of the regeneration. At the end of this stage, the cathode component thus regenerated is used. The results are listed together in the following table, where: the time (hours) represents the duration of the purification cycle, the current (mA) corresponds to the applied current in order to maintain the effectiveness of the purification treatment (output of the metallic content), the voltage (V) corresponds to the voltage measured at the electrode, the hydraulic head is expressed in 50% soda cm, the treated flow rate is expressed in 1 / h m2, - the contents of iron and nickel they are expressed in mg / kg of solution.

It was discovered that at the end of the purification cycle, the current must be increased in order to preserve effectiveness, due to the increase in the amount of metal deposited on the electrode.

CYCLE 2 after regeneration.

CYCLE 3 after regeneration It was discovered that the electrode can be regenerated and can retain its purification capacity. EXAMPLE 2 1 / Preparation of the cathodic component The procedure is as in the preceding example, except for the fact that the first suspension has the following composition: 30 g of PTFE fibers pretreated as shown in Example 1, - 70 g of carbon fibers, - 15 g of PTFE in the form of latex, - 50 g of precipitated silica, - 9 g of xanthan gum. The second suspension has the following composition: - 100 g of PTFE fibers pretreated as shown in Example 1, - 5 g of carbon fibers, - 20 g of PTFE in the form of latex, - 30 g of precipitated silica, - 3.6 g of triton X 100. 2 / Purification of the soda The treated effluent is soda at a concentration of 50% previously desalted as shown in Example 1. The treatment is carried out at a temperature of 50 ° C. The results obtained appear together in the following table:

Claims (10)

1. CLAIMS 1. Process for purification of a solution whose pH is higher than 14, in order to eliminate metallic impurities, characterized in that the solution is treated in an electrolysis cell, where the cathode includes a fibrous sheet obtained from a fiber mixture, wherein at least a fraction consists of electrically conductive fibers and a chosen agglomerator of fluoropolymers, wherein the fibrous sheet is deposited on a porous electrically conductive support.
2. 2. The process according to claim 1, characterized in that the solution is an alkali metal hydroxide solution.
3. 3. Process according to any of the preceding claims, characterized in that the cathode is able to be obtained by performing the following steps: [a] an aqueous suspension is prepared including a mixture of fibers, where at least one fraction consists of fibers electrically conductive, a chosen agglutinator of fluoropolymers, a pore-forming agent, and if appropriate, additives; [b] the suspension is deposited on a porous support by filtration under a programmed vacuum, [c] the thus obtained sheet is drained and optionally dried; [d] the resulting unit is optionally agglomerated at a temperature greater than or equal to the melting or softening temperature of the agglomerator; [e] the pore-forming agent is removed by a treatment carried out before or after the use of the cathode, or when the latter is used.
4. 4. The process according to any of the preceding claims, characterized in that the cathode is further used in combination with a diaphragm.
5. 5. The process according to any of the preceding claims, characterized in that the cathode is capable of being obtained by performing the following steps: [a] an aqueous suspension is prepared including a mixture of fibers, where at least one fraction consists of of electrically conductive fibers, a chosen agglutinator of fluoropolymers, a pore-forming agent and additives; [b] the suspension is deposited on a porous support by filtration under a programmed vacuum, [c] the thus obtained sheet is drained and optionally dried; [d] the resulting unit is optionally agglomerated at a temperature greater than or equal to the melting or softening temperature of the agglomerator; [e] the pore-forming agent is optionally removed; [f] an aqueous dispersion is deposited, including a mixture of organic and inorganic fibers, an agglomerator selected from the fluoropolymers, a pore-forming agent and, if appropriate, in the sheet thus formed; [g] the unit thus formed is dried and drained; [h] the unit is agglomerated at a temperature greater than or equal to the melting or softening temperature of the agglomerator; [i] the pore-forming agent is removed by a treatment formed before use of the cathode, or when the latter is used. •
6. 6. Process according to any of claims 1 to 3, characterized in that the cathode is used in combination with a membrane.
7. 7. Cathode including a fibrous sheet obtained from a mixture of carbon fibers, compounds based on cellulose fibers, and cationic polymers as in the case of cationic starch.
8. 8. Cathode according to the preceding claim, including a fibrous sheet obtained from a mixture of carbon fibers and compounds based on cellulose fibers pretreated with a cationic polymer, as in the case of a cationic starch.
9. 9. Cathode according to any of the Claims 7 and 8, characterized in that the mixing of the fibers additionally includes a pore-forming agent such as silica, as well as a surfactant.
10. 10. Cathode according to any of claims 7 to 9, characterized in that it is used in combination with a diaphragm or a membrane.